There is evidence of increased transcapillary permeability in diabetes. For example, increased urinary excretion by human diabetics of plasma proteins of mol. wt. greater than 40,000 has been reported, but among the proteins of greater than 40,000 mol. wt. there was no distinctive relationship between mol. wt. and the relative quantities excerted. Increased extravascular escape of albumin into the cerbral cortex of the diabetic rat has also been reported, but in this case that was an effect of molecular size since escape of larger proteins from the circulation was not affected by diabetes. WE propose to measure the accessability of the subendothelial basement membrane (BM) of the retina to circulating anti-BM antibody as an indicator of transcapillary permeability in normal and diabetic rats. We have large amount of high-titer BM-specific antiserum available for this project. To evaluate the influence of molecular size, both intact antibody and the monovalent Fab' fragment produced by papain digestion will be used. The low mol. wt. fragment will be 3H- labeled and the intact immunoglobulin 14C-labeled by reductive methylation with (3H)- NaBH4 and (14C) formaldehyde, respectively. Because of the unique nature of these probes and their ability to bind with high affinity at the site of extravasation, it will be possible to extract the harvested retinae with 0.5 percent Triton X-100, a non-denaturing detergent, to remove non-specifically-bound radioactivity such as radioactivity remaining in the lumen of capillaries. This will ensure specificity and low backgrounds in the normal or control animals. Combined with the dual labeled, dual mol. wt. protocol, these probes may prove exceptionally sensitive for detecting changes under baseline conditions and after challenge with agents that may later vascular permeability, such as increased plasma osmolarity, or exposure to circulating neuraminidase.